Re: some newbie questions about lpeg

["Duncan Cross" <duncan.cross@...>]

On 9/17/07, Roberto Ierusalimschy <roberto@inf.puc-rio.br> wrote:
> > On 9/16/07, gary ng <garyng2000@yahoo.com> wrote:
> > > What would be the equivalent of pattern{x,y} in regex,
> > > i.e. pattern can occur between x and y times ?
> >
> > First attempt:
> >
> > function lpeg.minmax(patt,min,max)
> >    return #(lpeg.P(patt)^min) * lpeg.P(patt)^-max
> > end
>
> Second attempt: (It is much more work, but the result is more efficient,
> as it does not read the patterns twice.)
>
> local function mult (p, n)
>   local np = m.P(true)
>   while n >= 1 do
>     if n%2 >= 1 then np = np * p end
>     p = p * p
>     n = n/2
>   end
>   return np
> end
>
> local function minmax (p, min, max)
>   if max == true then return p^min
>   else
>     max = (max == false) and min or max
>     if max < min then return m.P(false) end
>     local r = mult(p, min)
>     if max > min then r = r * p^(min - max) end
>     return r
>   end
> end
>
> (max == true means no maximum; max == false means exactly min repetitions)
>
> (The next version of 're' will support this syntax.)
>
> -- Roberto
>

You could even replicate the {x,y} syntax directly on patterns:

getmetatable(lpeg.P(1)).__call = function(p,t)  return minmax(p,
unpack(t));  end

(Not necessarily recommending this...)